Modern digital electronic equipment has the capability for communications at very high bit rates. When this high speed equipment communicates with low speed equipment a transmission disparity exists. This problem also exists when high speed equipment interfaces with low speed telephone facilities. Typically a data rate buffer is used as an interface between equipment having differing data rates. Additionally, data rate buffers are used to interface between facilities which are not in synchronization with each other.
Typically the communications between equipment having different bit rates is bidirectional (full duplex) and requires buffering in each direction. Thus, a bidirectional data rate buffer is used when high speed burst data signals are converted to and from low frequency data signals.
Prior art techniques for interfacing low speed data signals to high speed data signals include the use of data rate buffers for speed expansion and compression. Prior art techniques also include the use of pulse stuffing to achieve the conversion of asynchronous to synchronous signals. Typical of the prior art is U.S. Pat. No. 3,825,899 issued to H. H. Haeberle et al on July 23, 1974. The Haeberle et al patent teaches the use of compression and expansion buffers not only for burst data formation but also for pulse stuffing.
When the bidirectional communications between equipment having different data rates is required to be simultaneous (in bit synchronism) additional buffering problems exist. These problems are further compounded when this bit synchronism is to be maintained when the length of the high speed burst data may vary from one word to the next.
The prior mentioned Haeberle et al patent does not teach compression and expansion (data rate) buffers which operate interactively to transmit and receive in bit synchronism when the number of bits per data burst is variable.
In one typical data rate buffer application a polling centralized controller controls real-time information transfer by using a scanning operation to gather status and to distribute control commands to the slave stations. The communication links between the controller and slave stations typically operate at high data rates compared to the data rate transmission capability of the telephone facility. When the slave station is nearby (within 1000 feet of the controller) high data rate signals can be sent over the telephone facility. Often, however, it is necessary to connect a central controller with remotely located slave stations over conventional telephone lines. In such situations problems occur because of the data rate transmission disparity between the controller and the transmission facility.
One solution to such transmission rate incompatability problems is the use of data buffers at the interface between the high speed and low speed facilities. However, in some situations the problem is compounded in that it is also necessary to maintain full synchronism between the controller and the station. One such system where such a problem would exist for remote stations is described in the pending patent application of Fenton et al, Ser. No. 847,216 filed on Oct. 31, 1977, and incorporated by reference herein. In the Fenton et al application multibutton electronic telephone (MET) sets are periodically polled using high speed burst data signals from a central switching controller to determine the status of buttons and control lamps of the MET sets. The central switching controller is connected to the remotely located MET sets via telephone lines.
In the Fenton et al application the central switching controller polls each MET set in a bit synchronous manner such that a bit is simultaneously received from the MET set for each bit of the poll word transmitted to the MET set. The MET sets have a capability for operating in a partial scan (partial data word) mode or full scan (full data word) mode. Thus, the central controller may scan in an interactive manner, such that the length of the scan word to each MET set may either vary with time or vary with the prior response received from the MET sets. Since the number of MET sets, polled by the central controller may vary from one polling cycle (or frame) to the next, it would be desirable if the system could accommodate such changes in the number of words per frame cycle.
Accordingly, it is a problem to design a bidirectional data rate buffer which converts high speed burst data signals to low speed data signals without losing bit synchronism with either the high speed apparatus or the low speed apparatus when the number of binary bits in the high speed burst signal changes with each frame.
Additionally, it is a problem to design a data rate buffer which can accommodate changes in the number of high speed burst signals per frame.